1. Field of the Invention
The present invention is directed to apparatus and methods particularly suitable for precise aiming and delivery of magnetic stimulation, and more specifically, transcranial magnetic stimulation.
2. Description of the Related Art
Transcranial magnetic stimulation (“TMS”) is a means of repetitively stimulating the human brain through an intact scalp and skull, i.e., non-invasively. TMS is delivered by passing a brief (200 microsecond), strong (10,000 volts, 6,000 amps) electrical current through a coil of wire (a TMS stimulator) placed adjacent to the head. The passage of electrical current induces a strong (2 Tesla) magnetic field which, in turn, induces electrical currents in nearby tissues. In the case of nerve cells, if the induced current is sufficiently intense and properly oriented, it will result in synchronized depolarization of a localized group of neurons (i.e., neuronal “firing”). Initially, magnetic stimulation was used only for peripheral nerves, in which instance it is affecting nerve fibers rather than neuronal cell bodies. More recently (Barker et al., 1985), magnetic stimulation has shown to be able to depolarize neurons in the brain. The cellular element of the brain being affected by TMS was assumed, but not proven, to be fibers rather than neuronal cell bodies.
TMS has several present and potential applications, in the domains of basic neuroscience research and of the treatment of brain disorders. Applications for neuroscience research include, for example: imaging brain connectivity (e.g., Fox et al., 1997); establishing inter-regional and inter-hemispheric conduction times (e.g., Meyer et al., 1995); testing the function of specific brain areas by means of transient functional disruptions, so-called “virtual lesions” (e.g., Shipley & Zeki, 1995); and, studying the modification of synaptic efficacy induced by repetitive stimulation, termed LTP (long-term potentiation) and LTD (long-term depression). Potential clinical applications include, for example: pre-operative mapping, e.g., of language related brain areas (Epstein (et al., 1996); testing for neuronal conduction delays due to dysmyelinating disorders; and, treating brain disorders by selective modification (up or down regulation) of the synaptic efficacy of pathways (i.e., by inducing LTP and LTD; Wang, Wang and Scheich, 1996).
At present, TMS delivery is crude. The TMS effector or stimulator (commonly referred to as a “TMS coil”) is a wire-wound coil, typically shaped like a “B.” The B-shaped coil is placed against the scalp and held in place by a human operator. For the primary motor cortex and primary visual cortex (small sections of the total brain surface), proper positioning is established by the elicited response: muscle contractions when stimulating the primary motor cortex; illusory lights (phosphenes) when stimulating the primary visual cortex. In both of these areas, the effects are very sensitive to coil position and orientation.
For brain regions in which proper positioning cannot be determined by the induced effects (i.e., muscle contractions or subjective experience), position is generally determined by reference to a traditional pattern used for placement of EEG electrodes (10/20 system). The 10/20 system is based on scalp/skull landmarks which do not bear a reliable relationship to the functional anatomy of the brain. Further, when using the 10/20 system, there has been no strategy enunciated for determining proper orientation of the coil. Thus, a reliable method for determining the proper position and orientation of TMS coil placement for brain areas lacking immediately observable feedback is needed.
Application of TMS during radionuclide imaging (using positron-emission tomography (“PET”) or single photon emission tomography (“SPECT”)) has two important uses. First, radionuclide imaging can be used to monitor the induced response, determining precise location and quantifying response magnitude. This is extremely important for testing aiming algorithms and for determining the effect of stimulation parameters, such as intensity, rate, duration and the like. Second, an important use of TMS is to map brain connectivity using radionuclide imaging. For both of these applications, hand-held TMS delivery is inappropriate, for at least three reasons. First, hand-held delivery is unsafe, unnecessarily exposing the experimentor to the radiation used for imaging. Second, hand-held delivery is positionally unstable, degrading image quality by small movements of the holder. Third, hand-held delivery is intrinsically inaccurate and imprecise.
Further, current coil designs for delivery of TMS have been mainly intuitive and somewhat crude. Typical coil designs consist of two loop figure eight type coils, for peripheral nerve and brain stimulation, four loop coils for peripheral nerve stimulation, and variations in angles of these. While attention is paid to coil inductance, it is only for simple circuits that this may be easily calculated.
The target field method has been used to produce minimum inductance cylindrical gradient coils for MRI (Turner, 1986) and has been adapted for bi-planar coils (Martens et al., 1991). Minimum power designs have also been presented (Bowtell et al.). However, such design methods have not been applied to the design of magnetic stimulation coils.
Various combinations of circular or rectangular coil shapes have been designed. Figure eight type designs appear to be the most common. Further, B-shaped and slinky-type coils have also been designed (Cadwell; Lin et al.). Sections of toroids (Davey, Epstein, Carbunaru) with magnetically permeable cores are also known, and appear to be an efficient design. However, none of these provide an extremely focused field penetration. Two and four wing coils with a straight section joined with curves for peripheral nerve stimulation have been designed. (Ruohonen et al.) However, these designs are largely limited by intuition.
Thus, fundamental limitations on the utility of TMS for research and treatment include a lack of methods for precise, automated aiming (positioning and orienting) and safe, rigid (i.e., non-human) holding of the TMS stimulator, as well as the poor suitability of present coils for TMS.